Communication method and apparatus using analog and digital hybrid beamforming

ABSTRACT

A communication method and apparatus using analog and digital hybrid beamforming are provided. The method includes receiving a first message including a measurement and selection condition for hybrid beamforming from a Base Station (BS), measuring channels of a plurality of BS transmission beams, selecting at least one BS transmission beam based on channel measurements, transmitting report information about the selected at least one BS transmission beam to the BS, receiving from the BS a second message, estimating an effective channel matrix for the selected final BS transmission beam according to the measurement and report condition, determining feedback information for digital beamforming of the BS based on the effective channel matrix, transmitting the determined feedback information to the BS, and receiving a data burst from the BS according to a Multiple Input Multiple Output (MIMO) mode and/or a configuration scheduled based on the feedback information.

PRIORITY

This application is a continuation application of prior application Ser.No. 13/892,044, filed on May 10, 2013, which claimed the benefit under35 U.S.C. §119(a) of a Korean patent application filed on May 10, 2012in the Korean Intellectual Property Office and assigned Serial No.10-2012-0049920 and of a Korean patent application filed on Aug. 29,2012 in the Korean Intellectual Property Office and assigned Serial No.10-2012-0094630, the entire disclosure of each of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a beamforming-based wireless mobilecommunication system. More particularly, the present invention relatesto a communication method and apparatus using analog and digital hybridbeamforming.

2. Description of the Related Art

The development trend of wireless communication systems is toward higherdata rates to satisfy ever increasing demands for wireless data traffic.For example, wireless communication systems are being developed towardincreased spectral efficiency based on communication schemes such asOrthogonal Frequency Division Multiple Access (OFDMA), Multiple InputMultiple Output (MIMO), and the like in order to increase data rates.

As demands for traffic have accelerated due to increased demands forsmartphones and tablet Personal Computers (PCs) and the resulting rapidgrowth of applications requiring a large amount of traffic, it isdifficult to satisfy the soaring demands for wireless data simply byincreasing spectral efficiency.

To avert the problem, recent interest has focused on a millimeter-wavewireless communication system. When wireless communication is providedin a millimeter-wave frequency band, propagation loss, such as path lossand reflection loss, is increased in view of the spectral nature of themillimeter-wave frequency band and the resulting shortened propagationdistance reduces service coverage. Therefore, the millimeter-wavewireless communication system may expand service coverage by mitigatingthe path loss of waves using beamforming and thus increasing thepropagation distance of the waves.

The two types of beamforming schemes are digital beamforming (orTransmit (Tx) pre-Inverse Fast Fourier Transform (pre-IFFT)beamforming/Receive (Rx) post-Fast Fourier Transform (post-FFT)beamforming) and analog beamforming (or Tx post-IFFT beamforming/Rxpre-FFT beamforming). Digital beamforming uses a plurality of RadioFrequency (RF) paths based on Multiple Input Multiple Output (MIMO) anda digital precoder or codebook in the digital domain, and analogbeamforming uses a plurality of analog/RF devices (e.g., a phaseshifter, a Power Amplifier (PA), and a Variable Gain Amplifier (VGA))and an antenna configuration. While digital beamforming requires anexpensive Digital to Analog Converter (DAC) or Analog to DigitalConverter (ADC) and increases implementation complexity in order toincrease a beamforming gain, analog beamforming has limitations in termsof efficient use of frequency resources or maximization of beamformingperformance.

Since a wavelength is shortened in a millimeter-wave band, analogbeamforming using an antenna array with a number of antenna elementsarranged in a small space, such as a Uniform Linear Array (ULA) or aUniform Planar Array (UPA), is suitable. However, the analog beamforminghas limitations in its effectiveness in terms of efficient use ofresources, the increase of user or system throughput through MIMOschemes such as Single User MIMO (SU-MIMO), Multiple User MIMO(MU-MIMO), or spatial multiplexing, and the increase of Signal to NoiseRatio (SNR) or reliability through diversity or additional digitalbeamforming, as described before.

Accordingly, there exists a need for supporting a hybrid beamformingwhich is a combination of analog beamforming and digital beamforming,for efficient MIMO/beamforming.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and apparatus of transmitting andreceiving a signal in a communication system.

Another aspect of the present invention is to provide a method andapparatus of transmitting and receiving a signal by beamforming in amillimeter-wave communication system.

Another aspect of the present invention is to provide a hybridbeamforming structure by combining analog beamforming and digitalbeamforming for use in signal transmission and reception between aMobile Station (MS) and a Base Station (BS).

Another aspect of the present invention is to provide a method andapparatus of efficiently performing hybrid beamforming by selecting oneor more best beams from one or more analog beam sets having directivityon uplink/downlink between an MS and a BS and operating a digitalMultiple Input Multiple Output (MIMO)/beamforming efficiently using abeam pair based on the selected beams.

In accordance with an aspect of the present invention, a communicationmethod using analog and digital hybrid beamforming is provided. Thecommunication method includes receiving a first message including ameasurement and selection condition for hybrid beamforming from a BS,measuring channels of a plurality of BS transmission beams according tothe measurement and selection condition, selecting at least one BStransmission beam based on channel measurements according to themeasurement and selection condition, transmitting report informationabout the selected at least one BS transmission beam to the BS,receiving from the BS a second message including a measurement andreport condition for digital beamforming of a final BS transmission beamselected by the BS, estimating an effective channel matrix for theselected final BS transmission beam according to the measurement andreport condition, determining feedback information for digitalbeamforming of the BS based on the effective channel matrix,transmitting the determined feedback information to the BS, andreceiving a data burst from the BS according to a MIMO mode and/or aconfiguration scheduled based on the feedback information.

In accordance with another aspect of the present invention, acommunication method using analog and digital hybrid beamforming isprovided. The communication method includes transmitting to a MS a firstmessage including a measurement and selection condition for hybridbeamforming of a plurality of BS transmission beams, receiving from theMS report information about at least one BS transmission beam selectedaccording to the measurement and selection condition by the MS,selecting a final BS transmission beam for the MS based on the reportinformation, transmitting to the MS a second message including ameasurement and report condition for digital beamforming of the final BStransmission beam, receiving feedback information for digitalbeamforming of the BS from the MS, performing hybrid beamformingscheduling for the MS based on the feedback information, andtransmitting a data burst to the MS according to a scheduled MIMO modeand/or configuration.

In accordance with another aspect of the present invention, an MS forperforming communication using analog and digital hybrid beamforming isprovided. The MS includes a digital beamformer, an analog beamformer,and a controller configured to control the digital beamformer and theanalog beamformer. The controller is configured to receive a firstmessage including a measurement and selection condition for hybridbeamforming from a BS, to measure channels of a plurality of BStransmission beams according to the measurement and selection condition,to select at least one BS transmission beam based on channelmeasurements according to the measurement and selection condition, totransmit report information about the selected at least one BStransmission beam to the BS, to receive from the BS a second messageincluding a measurement and report condition for digital beamforming ofa final BS transmission beam selected by the BS, to estimate aneffective channel matrix for the selected final BS transmission beamaccording to the measurement and report condition, to determine feedbackinformation for digital beamforming of the BS based on the effectivechannel matrix, to transmit the determined feedback information to theBS, and to receive a data burst from the BS according to a MIMO modeand/or a configuration scheduled based on the feedback information.

In accordance with another aspect of the present invention, a BS forperforming communication using analog and digital hybrid beamforming isprovided. The BS includes a digital beamformer, an analog beamformer,and a controller configured to control the digital beamformer and theanalog beamformer. The controller is configured to transmit to an MS afirst message including a measurement and selection condition for hybridbeamforming of a plurality of BS transmission beams, to receive from theMS report information about at least one BS transmission beam selectedaccording to the measurement and selection condition by the MS, toselect a final BS transmission beam for the MS based on the reportinformation, to transmit to the MS a second message including ameasurement and report condition for digital beamforming of the final BStransmission beam, to receive feedback information for digitalbeamforming of the BS from the MS, to perform hybrid beamformingscheduling for the MS based on the feedback information, and to transmita data burst to the MS according to a scheduled MIMO mode and/orconfiguration.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram of a physical layer of a Base Station (BS)transmitter to support beamforming according to an exemplary embodimentof the present invention;

FIG. 2 is a block diagram of a physical layer of a BS transmitter tosupport beamforming according to an exemplary embodiment of the presentinvention;

FIGS. 3A and 3B are block diagrams of hybrid Transmission (Tx) andReception (Rx) beamforming structures according to an exemplaryembodiment of the present invention;

FIG. 4 illustrates communication between a BS and a Mobile Station (MS)according to an exemplary embodiment of the present invention;

FIG. 5 illustrates exemplary analog beams formed in different directionsin order to cover a specific cell/sector area in a beam space accordingto an exemplary embodiment of the present invention;

FIG. 6 illustrates an analog and digital hybrid beamforming procedureaccording to an exemplary embodiment of the present invention;

FIGS. 7A and 7B are block diagrams of hybrid Tx and Rx beamformingstructures according to an exemplary embodiment of the presentinvention;

FIG. 8 is a flowchart illustrating a scheduling procedure for hybridMultiple Input Multiple Output/BeamForming (MIMO/BF) according to anexemplary embodiment of the present invention; and

FIG. 9 is a flowchart illustrating a measurement and feedback procedurefor hybrid MIMO/BF according to an exemplary embodiment of the presentinvention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

Exemplary embodiments of the present invention will be provided toaddress the above-described technical aspects of the present invention.In an exemplary implementation, defined entities may have the samenames, to which the present invention is not limited. Thus, exemplaryembodiments of the present invention can be implemented with same orready modifications in a system having a similar technical background.

Transmit (Tx) beamforming increases directivity by focusing signal wavesin a specific direction through a plurality of antennas. A set ofantennas is called an antenna array and the individual antennas of theantenna array are called antenna elements. The antenna array may takevarious forms including a linear array, a planar array, and the like. Txbeamforming may increase a propagation distance by increasing signaldirectivity. Since signals are seldom transmitted in directions otherthan a specific direction, Tx beamforming may also significantly reducesignal interference with users other than an intended user.

A receiver may also perform Receive (Rx) beamforming using an Rx antennaarray. Since Rx beamforming increases the reception sensitivity ofsignals from a specific direction by focusing signal reception in thespecific direction, excluding signals from the other directions, Rxbeamforming may block signals causing interference.

A millimeter-wave wireless mobile communication system employsbeamforming to mitigate high propagation path loss in a millimeter-wavefrequency band. Furthermore, beamforming is needed in every case toreduce an unbalance between data and a control signal. The beamformingprocess as suggested by, for example, the Institute of Electrical andElectronics Engineers (IEEE) 802.1 lad standard includes two phases:Sector Level Sweep (SLS) and Beam Refinement Protocol (BRP).

IEEE 802.1 lad is a Wireless Local Area Network (WLAN) standard thatprovides a very small service area with a radius of 10 to 20 meters inthe 60-GHz millimeter-wave band. To overcome a wave propagation problemencountered with the millimeter-wave band, beamforming is used.

During the SLS phase, a Station (STA) that will perform beamformingtransmits the same sector frame repeatedly in different directions and apeer STA receives sector frames through quasi-omni antennas andtransmits feedback regarding a direction having the highest sensitivity.Therefore, the STA may perform beamforming by acquiring informationabout the direction having the highest sensitivity from the peer STA.

During the BRP phase, Tx and Rx beam directions between the two STAs arefine-adjusted after the SLS phase in order to increase Tx and Rxbeamforming gains. Typically, after the two STAs detect the best Tx beamduring the SLS phase, they search for the best Rx beam matching the bestTx beam during the BRP phase. Additionally, the Tx-Rx beam pair may befurther fine-adjusted by repeating the SLS and BRP phases.

Compared to the millimeter-wave wireless communication system, existing2nd Generation (2G) to 4th Generation (4G) cellular communicationsystems are designed to transmit and receive control channels and datain a sub-1 GHz or 1 to 3 GHz frequency band in an isotropic oromni-directional fashion. However, some resources are optionallyallocated to a user satisfying a specific channel condition by digitalbeamforming.

Research has been conducted to achieve an additional performance gain byutilizing the multipath propagation characteristics of channels withTx/Rx diversity based on multiple transmission and reception antennas,such as Multiple Input Multiple Output (MIMO), in the existing cellularsystems.

Meanwhile, the multipath propagation of channels is mitigated due to theafore-described channel characteristics and use of Tx/Rx beamforming inan extremely high frequency band like a millimeter-wave band. Therefore,a beamforming gain may be achieved but it is difficult to support Tx/Rxdiversity. Accordingly, previous studies were limited to determinationof a beamforming weight coefficient that optimizes a performance indexsuch as Signal to Noise Ratio (SNR) by maximizing a beamforming gainduring beamforming.

Wireless Gigabit (WiGig), which does not support MIMO, is implementedbased on beamforming through an analog array of a plurality of RadioFrequency (RF)/antenna elements, basically in one RF path or RF chain.For beamforming, a transmitter sweeps beams of a specific beam patternin a plurality of directions and a receiver selects a beam having thelargest signal strength and transmits feedback about the selected beamto the transmitter. This technique is generally applicable to an indoorenvironment having a Line of Sight (LoS) channel path in a short rangeof a few meters without mobility. In an outdoor wireless mobilecommunication environment characterized by mobility of tens ofkilometers per hour, fast terminal switching, obstacle-incurred Non-LoS(NLoS) path characteristics, or a rapidly changing channel state causedby channel fading, beamforming that forms narrow beams havingdirectivity, maximizing a beam gain in a specific direction may onlyincrease sensitivity due to performance degradation attributed to theuser environment.

Another MIMO/BF technique is digital beamforming used in 3rd GenerationPartnership Project (3GPP) Long Term Evolution (LTE) and WorldwideInteroperability for Microwave Access (WiMAX). Although the digitalbeamforming scheme supports MIMO/BF with fewer than eight streams basedon digital precoding, it is difficult to apply the digital beamformingas a millimeter-wave band because of hardware complexity and largesignal overhead in supporting multiple RF paths. In addition, a WiGiganalog beamforming scheme has a limited performance due to use of asingle beam without supporting beam sweeping and MIMO, and suffers fromperformance degradation in a multi-path channel environment.

Hybrid beamforming applicable to a cellular communication systemaccording to an exemplary embodiment of the present invention combinesanalog beamforming that overcomes path loss by high-gain beamforming atan RF end with digital beamforming that maximizes capacity by MIMO.

A beamforming-based millimeter-wave wireless mobile communication systembecomes sensitive to channel fading and obstacles due to largepropagation loss, large penetration loss, and low multi-path propagationinherent to the channel propagation nature of a millimeter-wave band andstrong directivity resulting from beamforming.

An analog and digital hybrid beamforming structure for transmission andreception between a Base Station (BS) and a User Equipment (UE) selectsone or more best beams from one or more analog beam sets havingdirectivity on downlink and uplink and performs digital MIMO/BFefficiently using the selected beams in combination. Therefore, a largepropagation loss in the millimeter-wave band is mitigated, andperformance such as channel capacity or diversity is maximized byadditional use of a MIMO/BF scheme.

For this purpose, a hybrid beamforming mode is selected in an exemplaryembodiment of the present invention. A BS transmits reference signals inmulti-directional beams that cover beam spaces and indicates abeam-space analog beam selection mode to an MS. The mode selection maybe based on the state of the MS in a connected mode. The MS selects oneor more analog beams (i.e., BS Tx beams) by beam space sweeping andmeasurement and reports information about the selected one or moreanalog beams to the BS.

In an exemplary implementation, the BS indicates a selected Tx analogbeam to the MS and the MS estimates a beam spatial channel matrix (or aneffective channel matrix) made up of beamforming coefficients for theselected analog beam. Subsequently, the BS and/or the MS selects a MIMOmode and a precoder based on the beam spatial channel matrix. The MS mayselect the precoder using a given codebook or the BS may determine aprecoding matrix based on the beam spatial channel matrix reported bythe MS.

FIG. 1 is a block diagram of a physical layer of a BS transmitter thatsupports beamforming according to an exemplary embodiment of the presentinvention. The physical layer is configured as a hybrid structure thatuses analog beamforming and digital beamforming simultaneously, by wayof example.

Referring to FIG. 1, L digital signals corresponding to L layers areprovided to a MIMO encoder 120 through encoders (ENCs) 110-1 to 110-Land modulators (MODs) 115-1 to 115-L. A precoder 125 converts M_(t)streams received from the MIMO encoder 120 to N_(f) precoded signalscorresponding to N_(f) RF paths. The precoded signals are provided to ananalog beamformer 150 in RF paths including Inverse Fast FourierTransforms (IFFTs) 130-1 to 130-N_(f), Parallel-to-Serial (P/S)converters 135-1 to 135-N_(f), Cyclic Prefix (CP) inserters 140-1 to140-N_(f), and Digital to Analog Converters (DACs) 145-1 to 145-N_(f).

The analog beamformer 150 at the rear ends of the DACs 145-1 to145-N_(f) includes a plurality of frequency converters or mixers, aplurality of phase shifters, and a plurality of Power Amplifiers (PAs)or Variable Gain Amplifiers (VGAs) corresponding to the respective RFpaths and forms beams to be transmitted in a specific direction bycontrolling the phases and amplitudes of signals input to a plurality ofantenna elements in the RF paths. The beams are transmitted through anantenna array 155 that is formed by grouping a plurality of antennaelements to increase a beamforming gain.

Digital beamforming through RF paths including the IFFTs 130-1 to130-N_(f), the MIMO encoder 120, and the precoder 125 before the DACs145-1 to 145-N_(f) enables achievement of an additional beamforminggain, Multi Unit-MIMO (MU-MIMO), frequency selective allocation, andmulti-beam forming. Of course, it is to be understood that the hybridbeamforming structure illustrated in FIG. 1 may be modified in variousmanners by modifying and/or combining a plurality of blocks.

One or more beams having different beam widths and beam gains that aregenerated from the hybrid beamforming structure may be used in differentmanners according to the channel characteristics of a reference signal,a data channel, and/or a control channel, the mobility and channelcharacteristics of an MS, uplink/downlink, or transmission/reception. Aselected beam is formed by controlling the beamforming coefficient of ananalog or digital end so that the beam has a specific beamwidth and beamgain in a specific direction. If the same input power is set for theantenna elements, a maximum beam gain in the steered direction of thebeam becomes smaller by setting a wider beamwidth.

FIG. 2 is a block diagram of a physical layer of a BS transmitter tosupport beamforming according to an exemplary embodiment of the presentinvention.

Referring to FIG. 2, L digital signals corresponding to L layers areprovided to a MIMO encoder 220 through encoders (ENCs) 210-1 to 210-Land modulators (MODs) 215-1 to 215-L. A precoder 225 converts M_(t)streams received from the MIMO encoder 220 to N_(f) precoded signalscorresponding to N_(f) RF paths. The precoded signals are provided to ananalog beamformer 250 in RF paths including IFFTs 230-1 to 230-N_(f),P/S converters 235-1 to 235-N_(f), CP inserters 240-1 to 240-N_(f), andDACs 245-1 to 245-N_(f).

The analog beamformer 250 forms beams to be transmitted in a specificdirection through a plurality of phase shifters and a plurality of PAsor VGAs. The beams are transmitted through N_(t) antenna array groups255-1 to 255-N_(t). In the hybrid beamforming structure illustrated inFIG. 2, the N_(f) RF paths correspond to the N_(t) antenna array groups.

FIGS. 3A and 3B are block diagrams of hybrid Tx and Rx beamformingstructures according to an exemplary embodiment of the presentinvention.

Referring to FIGS. 3A and 3B, a transmitter 300 includes a digitalbeamformer 310 and an analog beamformer 330. The digital beamformer 310is connected to the analog beamformer 330 through a plurality of RFpaths including IFFTs 320, P/S converters 325, and DACs 327.

The digital beamformer 310 includes a MIMO encoder 312 and a BaseBand(BB) precoder 314. The analog beamformer 330 includes frequencyconverters 332, phase shifters/PAs 334, and antenna arrays 336 in therespective RF paths. Analog beams are formed in the antenna arrays 336mapped to the respective RF paths (i.e., RF chains). While not shown,the transmitter 300 may further include a controller to control thedigital beamformer 310 and the analog beamformer 330, to acquireinformation needed for hybrid beamforming, to exchange the informationwith a receiver, and to determine information needed to control thedigital beamformer 310 and the analog beamformer 330, for example, abeamforming coefficient matrix.

Beams formed by the transmitter 300 are transmitted to a receiver 305 onbeam-spatial effective channels established over MIMO channels (H) 340between multiple channels of the transmitter and the receiver. Similarlyto the transmitter 300, the receiver 305 includes an analog beamformer350 and a digital beamformer 370. The analog beamformer 350 is connectedto the digital beamformer 370 through a plurality of RF paths includingAnalog to Digital Converters (ADCs) 360, Serial to Parallel (S/P)converters 362, and Fast Fourier Transforms (FFTs) 364. The analogbeamformer 350 includes antenna arrays 352, Low Noise Amplifiers(LNAs)/phase shifters 354, and frequency converters 356 in therespective RF paths. The digital beamformer 370 includes a BB combiner372 and a MIMO decoder 374.

While not shown, the receiver 305 may further include a controller tocontrol the digital beamformer 370 and the analog beamformer 350, toacquire information needed for hybrid beamforming, to exchange theinformation with the transmitter 300, and to determine informationrequired to control the digital beamformer 370 and the analog beamformer350, for example, a beamforming coefficient matrix. The controllerdetermines the best analog beam by performing channel estimation onanalog beams received from the antenna arrays 352 mapped to therespective RF paths (i.e., RF chains).

The transmitter 300 forms analog beams having directivity in differentdirections by analog beamforming and transmits and receives data withimproved performance in an analog Tx-Rx beam pair selected from amonganalog Tx and Rx beams by additional digital MIMO/BF processing.

FIG. 4 illustrates communication between a BS and an MS according to anexemplary embodiment of the present invention.

Referring to FIG. 4, a BS 400 manages a cell divided into one or moresectors as its service coverage area and forms a plurality of Tx/Rxbeams 412 using the afore-described digital and analog hybridbeamforming structure. The BS 400 transmits a plurality of beamformedsignals by sweeping them simultaneously or successively, as indicated byreference numeral 410.

An MS 420 located within the cell of the BS 400 may be configured toreceive signals omni-directionally without supporting Rx beamforming,receive signals while supporting Rx beamforming by using one beamformingpattern each time, or receive signals while supporting Rx beamforming bysimultaneously using a plurality of beamforming patterns in differentdirections.

If the MS 420 does not support Rx beamforming, the MS 420 measures thechannel quality of a reference signal in each transmission beam andreports the measurements to the BS 400. The BS 400 selects the best beamfor the MS 420 from among a plurality of Tx beams. If the MS 420 isconfigured to support Rx beamforming, the MS 420 measures the channelqualities of a plurality of Tx beams 422 received from the BS 400 foreach reception beam pattern and reports total or some high-rankedmeasurements of all Tx-Rx beam pairs to the BS 400. The BS 400 mayallocate an appropriate Tx beam to the MS 420. If the MS 420 is capableof receiving a plurality of Tx beams from the BS 400 or supporting aplurality of BS Tx-MS Rx beam pairs, the BS 400 may select a beam,taking into account diversity transmission through repeated transmissionor simultaneous transmission.

FIG. 5 illustrates exemplary analog beams formed in different directionsin order to cover a specific cell/sector area in a beam space accordingto an exemplary embodiment of the present invention. In the illustratedcase of FIG. 5, beams patterns are formed by use of a single antennaarray.

Referring to FIG. 5, a plurality of beams having predeterminedbeamwidths and predetermined beam gains may be formed with respect to abeam 500 in a reference direction (at an azimuth angle or elevationangle of 0).

FIG. 6 illustrates an analog and digital hybrid beamforming procedureaccording to an exemplary embodiment of the present invention. While aTime Division Duplex (TDD) frame divided into asynchronization/broadcasting channel region, Downlink (DL) subframes,and Uplink (UL) subframes is shown as an example in FIG. 6, it is to beunderstood that the hybrid beamforming procedure is applicable to aFrequency Division Duplex (FDD) frame or other frame structures in thesame manner or in a similar manner.

Referring to FIG. 6, a BS 600 forms one or more analog beams per RFchain or antenna array in different directions within a cell/sector andtransmits a reference signal such as a mid-amble or Channel StateInformation Reference Signal (CSI-RS) in each analog beam so that an MSmay measure the channel quality of an analog beam steered in eachdirection (e.g., a Carrier-to-Interference-and-Noise Ratio (CINRs),Received Signal Strength Indicator (RSSIs), etc.) in step 612. Thereference signals may be transmitted in N_(Tx) sub-time areas of apredetermined time area in a DL subframe.

N_(Tx) is the number of analog beams transmitted through each RF chainor antenna array from the BS. More specifically, the BS allocates thereference signals of different RF chains to be transmitted in one analogTx beam to different frequency resources in one sub-time area (e.g., asymbol duration). Therefore, for each RF chain, reference signals aretransmitted in analog Tx beams formed in different directions acrossN_(Tx) sub-time areas.

In step 614, an MS 605 measures the channels of the respective analogbeams from the reference signals, for each BS RF chain and selects oneor more BS RF chains or antenna arrays or one or more analog beams perBS RF chain or antenna array, which satisfy a specific threshold orcondition suitable for data transmission and reception between the BSand the MS based on channel qualities (e.g., CINRs, RSSIs, etc.)obtained from the measurements. For example, the MS 605 may select M BSanalog beams at maximum or M analog beams per BS RF chain or antennaarray at maximum according to a given value of M.

If the MS 605 also supports hybrid beamforming, the MS 605 may measurethe channel qualities of BS Tx-MS Rx analog beam pairs, while sweepingone or more MS Rx beams and may select one or more BS Tx analog beams orone or more BS Tx-MS Rx analog beam pairs based on the measurements. Acriterion or condition based on which a Tx/Rx analog beam is selectedmay be determined based on a threshold set by the MS 605 or a specificbeam selection metric and/or threshold indicated to all MSs or eachindividual MS within the cell by the BS 600.

The beam selection metric may be expressed as various forms. Forexample, the beam selection metric may include effective channels forone or more analog beams being carried through one or more RF chains andaverage, highest, or lowest SNR of respective MIMO streams calculated bymeasuring a channel matrix having the effective channels as entries. Inanother example, the beam selection matrix may be a channel capacitycalculated by a channel capacity formula.

In step 616, the MS 605 reports to the BS information indicating one ormore BS RF chains or antenna arrays that have been selected based on themeasurements of the channel qualities of all possible BS Tx-MS Rx analogbeam pairs, and analog beam index information indicating one or moreanalog Tx beams or one or more BS Tx-MS Rx analog beam pairs per RFchain or antenna array. The reported information may be transmitted, forexample, in a predetermined reporting area of a UL subframe. In step618, the BS 600 may downselect a final BS Tx analog beam(s) based on theanalog beam index information, taking into account scheduling. The BS600 transmits information about selected final BS RF chains or antennaarrays and information about a selected final analog Tx beam(s) per RFchain or antenna array to the MS 605 by a DL control message in a DLsubframe.

In step 620, the MS 605 estimates an effective channel matrix of a beamspace regarding the selected final BS Tx analog beams and best MS Rxanalog beams mapped to the BS Tx analog beams. The effective channelmatrix may be obtained by applying BS Tx analog beamforming and MS Rxanalog beamforming to a channel matrix between the Tx antennas of the BS600 and the Rx antennas of the MS 605 in an antenna space. For example,the effective channel matrix may be expressed as Equation (1).

y=W* _(BB,MS) W* _(RF,MS) HW _(RF,BS) W _(BB,BS) s+n=W* _(BB,BS) HW_(BB,BS) s+n

H=W* _(RF,MS) HW _(RF,BS) :N _(RF,MS) ×N _(RF,BS)   Equation (1)

In Equation (1), H represents an ideal channel matrix, H represents anestimated effective channel matrix, s represents a transmitted signal, yrepresents a received signal, and n represents noise. W_(BB) representsa beamforming coefficient matrix used in the digital beamformer andW_(RF) represents a beamforming coefficient matrix used in the analogbeamformer. Subscripts MS and BS represent beamforming coefficientmatrixes used respectively in the MS and the BS. N_(RF,MS) and N_(RF,BS)represent the numbers of RF paths in the MS and the BS, respectively.

The MS 605 estimates or predicts at least one of an optimum MIMO rank(or an optimum number of MIMO streams), an optimum MIMO precoder (orcodebook), an effective CINR (or Channel Quality Indication (CQI)), andan optimum Modulation and Coding Scheme (MCS) level for digitalbeamforming of the BS 600, using the estimated effective channel matrix,in step 620.

In an alternative exemplary embodiment, the MS 605 may determine asupported MIMO rank or the number of supported MIMO streams based on aneffective channel matrix made up of one or more BS RF chains or antennaarrays, analog beams for each of the BS RF chains or antenna arrays, oneor more MS RF chains or antenna arrays, and analog beams for each of theMS RF chains or antenna arrays. In addition, the MS 605 may select aprecoder or codebook that maximizes the afore-described beam selectionmetric based on the average, highest, or lowest of the SNRs of MIMOstreams or a channel capacity, which is estimated over each of specificTx precoders, predetermined codebooks, or Precoder Matrix Indexes (PMIs)for precoding in a transmitter. An effective CINR is estimated based onthe selected precoder or codebook.

The supported MIMO rank or the number of supported MIMO streams islimited by the maximum number of RF chains that the BS allocates to theMS, arbitrarily by the BS, or additionally by the number of RF chains orantenna arrays selected by the BS or the MS. In another exemplaryembodiment, the BS or the MS presets the supported MIMO rank or thenumber of supported MIMO streams. The MS may select a codebook or PMIthat optimizes transmission performance within the MIMO rank or thenumber of MIMO streams, estimate an effective CINR corresponding to theselected codebook or PMI, and feed back the estimated effective CINR tothe BS.

The MS 605 feeds back the estimated or predicted information to the BS600 in step 622. The BS 600 performs hybrid beamforming scheduling forthe MS 605 based on the feedback information and transmits a DL databurst to the MS 605 according to a scheduled MIMO mode and/or MCS levelin step 624.

In another exemplary embodiment of the present invention, the MS 605 mayfeed back information about the estimated effective channel matrix tothe BS 600 in step 622. The BS 600 may select an appropriate MIMOprecoder (or codebook) directly by performing Singular ValueDecomposition (SVD) of the effective channel matrix or a differentMIMO/BF scheme based on the feedback information and may performscheduling and data transmission in step 624. Herein, this technique offeeding back an effective channel matrix will be referred to as ananalog feedback of an effective channel matrix.

In another exemplary embodiment of the present invention, the MS 605 mayperform steps 614 through 620 simultaneously, instead of sequentiallyperforming the operation of selecting BS RF chains or antenna arrays andan analog beam per RF chain or antenna array or selecting a combinationof BS RF chains or antenna arrays, a Tx analog beam per BS RF chain orantenna array, MS RF chains or antenna arrays, and an Rx analog beam perMS RF chain or antenna array, the operation of selecting a MIMO rank orthe number of MIMO streams, and a precoder, codebook, or PMI, takinginto account digital MIMO precoding of the selected RF chains or antennaarrays and the selected Tx and/or Rx analog beams, and the operation ofestimating an effective CINR or CQI in steps 614 through 620. Morespecifically, the MS 605 may perform computation over every possiblecombination of a BS Tx RF chain or antenna array, Tx analog beams of theBS Tx RF chain or antenna array, an MS Rx RF chain or antenna array, andRx analog beams of the MS Rx RF chain or antenna array, for use inselecting the number of MIMO streams or a MIMO codebook. Substantiallysimultaneously, the MS 605 may select information such as RF chains orantenna arrays of the BS and the MS, analog beams for each of the RFchains or antenna arrays, the number of MIMO streams, and a digitalprecoder or codebook. The selected information is reported as an analogfeedback to the BS 600.

While the operations and procedures of an MS and a BS have beendescribed above in the context of DL hybrid beamforming, the MS and theBS may operate in a similar manner for UL hybrid beamforming byexchanging their roles in the DL hybrid beamforming.

FIGS. 7A and 7B are block diagrams of hybrid Tx and Rx beamformingstructures according to another exemplary embodiment of the presentinvention. In the hybrid Tx and Rx beamforming structures which areconfigured based on the hybrid beamforming structure illustrated in FIG.1, Tx and Rx analog beams having directivity are formed in differentdirections by analog beamforming between an MS and a BS and a Tx-Rxanalog beam pair selected from among the Tx and Rx analog beams issubjected to additional digital MIMO/BF processing. Therefore, datatransmission and reception are performed with improved performance.

Referring to FIG. 7A, a transmitter 700 includes a digital beamformer710 and an analog beamformer 730. The digital beamformer 710 isconnected to the analog beamformer 730 through a plurality of RF pathsincluding IFFTs 720, P/S converters 725, and DACs 727. The digitalbeamformer 710 includes a MIMO encoder 712 and a BB precoder 714. Theanalog beamformer 730 includes frequency converters 732 and phaseshifters/PAs 374 for the respective RF paths. Signals from the phaseshifters/PAs 734 are summed on an antenna element basis and provided tothe antenna elements of an antenna array 736. Analog beams are formed bythe single antenna array 736 shared by the plurality of RF paths (i.e.,RF chains).

While not shown, the transmitter 700 may further include a controller tocontrol the digital beamformer 710 and the analog beamformer 730, toacquire information needed for hybrid beamforming, to exchange theinformation with a receiver, and to determine information needed tocontrol the digital beamformer 710 and the analog beamformer 730, forexample, a beamforming coefficient matrix.

Beams formed by the transmitter 700 are transmitted to a receiver 705 onbeam-space effective channels established over MIMO channels H 740between multiple channels of the transmitter 700 and the receiver 705.

Referring to FIG. 7B, similarly to the transmitter 700, the receiver 705includes an analog beamformer 750 and a digital beamformer 770. Theanalog beamformer 750 is connected to the digital beamformer 770 througha plurality of RF paths including ADCs 760, S/P converters 762, and FFTs764. The analog beamformer 750 includes an antenna array 752 havingantenna elements corresponding to the RF paths, and LNAs/phase shifters754 and frequency converters 756 corresponding to the respective RFpaths. The digital beamformer 770 includes a BB combiner 772 and a MIMOdecoder 774.

While not shown, the receiver 705 may further include a controller tocontrol the digital beamformer 770 and the analog beamformer 750,acquire information needed for hybrid beamforming, exchange theinformation with the transmitter 700, and determine information requiredto control the digital beamformer 770 and the analog beamformer 750, forexample, a beamforming coefficient matrix. The controller performschannel estimation on analog beams output from the plurality of RF paths(i.e., RF chains) sharing the single antenna array and thus determinesthe best analog beam based on the channel estimation.

The transmitter 700 forms analog beams having directivity in differentdirections by analog beamforming and transmits and receives data withimproved performance in a Tx-Rx analog beam pair selected from among Txand Rx analog beams by additional digital MIMO/BF processing.

In another exemplary embodiment of the present invention, thetransmitter of FIG. 3A and the receiver of FIG. 7B may be used or thetransmitter of FIG. 7A and the receiver of FIG. 3B may be used. Inanother exemplary embodiment of the present invention, a transmitter anda receiver may be configured by modifying the hybrid beamformingstructure of FIG. 1 or FIG. 2.

Now a description will be given of an exemplary operation of an MS toselect an analog beam (i.e., a BS Tx beam) and estimate and select adigital precoder or codebook for additional digital MIMO/BF processingand an exemplary operation of a BS corresponding to the operation of theMS, when hybrid beamforming is implemented. While the followingdescription is given of a DL operation in which the BS transmits asignal and the MS receives a signal, substantially the same thing isapplicable to a UL operation.

The BS and the MS simultaneously or sequentially perform an operation ofselecting one or more analog beams (i.e., BS Tx beams) or one or more BSTx-MS Rx beam pairs for hybrid beamforming and an operation ofscheduling digital MIMO/BF for the selected one or more analog beams orBS Tx-MS Rx beam pairs.

FIG. 8 is a flowchart illustrating a scheduling procedure for hybridMIMO/BF according to an exemplary embodiment of the present invention.

Referring to FIG. 8, the BS broadcasts or unicasts information aboutbeamforming supported by the BS to the MS in step 802. The beamforminginformation includes analog beamforming information. The analogbeamforming information specifies, for example, at least one of thenumber of BS RF chains or antenna arrays, the number of BS Tx beams perBS RF chain or antenna array formed in different directions, the arraygain of each BS Tx beam, the number of beams supported simultaneously bythe BS, the number of beams supported simultaneously for one MS or oneburst, and the configuration of reference signals per BS Tx beam. Thebeamforming information may further include digital beamforminginformation for hybrid beamforming. In an exemplary embodiment of thepresent invention, if digital beamforming is used according to a digitalprecoder scheme based on a quantized codebook, the digital beamforminginformation includes information about a codebook set available to theMS. In another exemplary embodiment of the present invention, the analogbeamforming information may be broadcast or unicast to each MS withinthe cell and the digital beamforming information may be unicast to an MSthat has selected an analog beam pair.

In step 804, the BS transmits a request message requesting feedbackinformation for hybrid beamforming, particularly analog beamforming, toan MS that has performed or is performing network entry. The requestmessage may include a measurement/selection condition based on which anRF chain or antenna array, an analog beam, a digital MIMO precodingscheme, and the like are selected for hybrid beamforming.

For example, the measurement/selection condition includes at least oneof the number of BS Tx RF chains or antenna arrays, the (maximum) numberof BS Tx beams to be reported per BS Tx RF chain or antenna array, the(maximum) number of beam pairs to be reported, the (maximum) number ofBS Tx beams to be reported per MS Rx beam, and the (maximum) number ofMS Rx beams to be reported. In another example, themeasurement/selection condition includes at least one of the (maximum)number of MIMO streams, a channel quality condition used to select a BSTx beam or a BS Tx-MS Rx beam pair, and a specific MIMO mode about whichfeedback information is to be transmitted.

The channel quality condition may include, for beam selection, forexample, at least one of a signal strength threshold (e.g., CINR, RSSI,etc. threshold) CINR_(Th)/RSSI_(Th), a relative thresholdΔCINR_(th)/ΔRSSI_(Th), a standard deviation threshold σ_(Th)/σ_(RSSI), acorrelation threshold ρ_(Th), a combined CINR/RSSI threshold forselected multiple beam pairs, and a threshold for the difference betweena predetermined reference (e.g., the maximum CINR/RSSI of the selectedmultiple beam pairs) and the CINRs/RSSIs of the selected multiple beampairs. The specific MIMO mode may be at least one of Multiple InputMultiple Output-Spatial Multiplexing (MIMO-SM), Multiple Input MultipleOutput-Space Time Code (MIMO-STC), and MIMO/BF. Themeasurement/selection condition may further include a supported MIMOrank or the number of supported MIMO streams, set by the BS.

In step 806, the MS feeds back to the BS a report message includinginformation about one or more BS RF chains or antenna arrays and one ormore beam pairs (or BS Tx beams) that are selected based on themeasurement/selection condition, and Channel State Information (CSI)(e.g., RSSIs, CINRs, etc.) of the beam pairs (or BS Tx beams). The BSdownselects one or more BS RF chains or antenna arrays and one or morebeam pairs per BS RF chain or antenna array, taking into account itsscheduling criterion based on the information about the selected beampairs (or BS Tx beams) and the CSI about the beam pairs (or BS Tx beams)in step 808.

In step 810, the BS transmits a request message requesting feedbackinformation about the selected one or more beam pairs to the MS, fordigital beamforming. The request message includes information about theone or more BS RF chains or antenna arrays selected by the BS and one ormore beam pairs selected for each of the BS RF chains or antenna arrays.The request message may further include digital beamforming information.The digital beamforming information includes information about a channelmeasurement and feedback report condition for the selected beam pairs.For example, the channel measurement and feedback report conditionspecifies, for example, the maximum number of BS RF paths allocated tothe MS, the maximum number of MIMO streams, and feedback informationneeded for analog/digital/hybrid MIMO/BF.

In an exemplary embodiment of the present invention, if digitalbeamforming is used according to a digital precoder scheme based on aquantized codebook, the digital beamforming information includesinformation about a codebook set available to the MS. In anotherexemplary embodiment of the present invention, the BS and the MS maycommonly pre-store information about a codebook preset or predefinedaccording to a predetermined rule.

In an alternative exemplary embodiment of the present invention, thedigital beamforming information may include information indicatingwhether digital beamforming is to be performed based on an analogfeedback or a codebook. In another alternative exemplary embodiment ofthe present invention, the digital beamforming information may indicatea feedback to be received. For example, the digital beamforminginformation may specify at least one of an optimum MIMO mode estimatedby the MS, a preferred PMI representing an optimum digital precoder, anoptimum codebook, the number of MIMO streams, estimated CSI, anestimated MCS level, the elements of a channel matrix, and the channelmatrix or its equivalent.

Step 812 or step 814 is performed depending on whether beamforming isperformed based on a codebook or an analog feedback. The type of digitalbeamforming to be used may be determined in various manners includingnegotiations between the BS and the MS, an indication from the BS, arequest from the MS, the capabilities of the BS and the MS, a systemstandard, and the like.

In step 812, the BS receives feedback information for codebook-baseddigital beamforming from the MS. The feedback information includes atleast one of an optimum MIMO mode, a preferred PMI representing anoptimum digital precoder, an optimum codebook, the number of MIMOstreams, CSI, and an MCS level, which are estimated based on theselected beam pairs by the MS. In step 814, the BS receives feedbackinformation for analog feedback-based digital beamforming from the MS.The feedback information is information needed to reconfigure theelements of a channel matrix, the channel matrix, or an equivalent ofthe channel matrix. For example, the feedback information may include anormalized channel matrix, a decomposed channel matrix, or a channelcovariance matrix.

In step 816, the BS finally performs scheduling for hybrid beamformingby combining the feedback information for digital beamforming receivedin step 812 or 814 with the feedback information for analog beamformingreceived in step 806. Thus, the BS determines a MIMO mode and itsrelated configuration, for example, selected beam pairs, MIMO streams, aPMI representing a selected digital precoder, and an MCS level. The BSallocates a data burst according to the selected/scheduled configurationand transmits the allocated data burst to the MS in step 818.

For example, if the BS receives feedback information about a channelmatrix for analog feedback-based digital beamforming from the MS, the BSestimates the channel capacities of different MIMO modes havingdifferent digital MIMO precoders (codebooks) over the selected beampairs, selects at least one MIMO mode having the largest channelcapacity (or satisfying a given condition), selects a preferred PMI(precoder or codebook) for digital beamforming of the selected beampairs, and estimates CSI (CSI, CQI, CINR, RSSI or MCS level), therebydetermining a configuration needed for digital beamforming in step 816.

In an alternative exemplary embodiment, after receiving the feedbackinformation in step 806, the BS may proceed directly to step 816 withoutrequesting feedback information for additional beam selection anddigital beamforming. In this case, the MS completes beam selection foranalog beamforming and estimation and configuration selection fordigital beamforming.

In another exemplary embodiment, the BS may request feedback informationfor both analog beamforming and for digital precoding in step 804, mayreceive the feedback information such as an RF chain or antenna arrays,analog beams of each of one or more selected RF chains or antennaarrays, a codebook/PMI and a MIMO rank for digital precoding by whichone or more selected Tx analog beams are combined prior to transmissionin step 806, and may proceed directly to step 816.

FIG. 9 is a flowchart illustrating a measurement and feedback procedurefor hybrid MIMO/BF according to an exemplary embodiment of the presentinvention.

Referring to FIG. 9, the MS receives beamforming information from the BSon a broadcast channel or a unicast channel in step 902. The beamforminginformation includes at least one of analog beamforming information anddigital beamforming information. For example, the analog beamforminginformation specifies the number of BS RF chains, the number of BSantenna arrays, the number of BS Tx beams per BS RF chain or antennaarray, and the configuration of reference signals per BS Tx beam.

The MS receives a feedback request for hybrid beamforming, particularlyanalog beamforming from the BS in step 904 and detects ameasurement/selection condition included in the feedback request in step906. The measurement/selection condition may include at least one of the(maximum) number of available BS RF chains or antenna arrays, the(maximum) number of available BS Tx beams, the (maximum) number ofavailable beam pairs, the (maximum) number of BS Tx beams per MS Rxbeam, the (maximum) number of MS Rx beams, the (maximum) number of MIMOstreams, a channel quality condition of selecting BS Tx beams or BSTx-MS Rx beam pairs, and a criterion for a specific MIMO mode.

The MS measures the channel qualities of BS Tx-MS Rx beam pairs (or BSTx beams) according to the measurement/selection condition set by the BSin step 908 and selects one or more beam pairs (or BS Tx beams) based onthe measurements in step 910.

Now a description will be given of an exemplary embodiment of selectinga beam pair in the MS.

Equation (2) expresses a channel matrix measured by the MS in step 908.

$\begin{matrix}{\overset{\_}{H} = \begin{bmatrix}{v_{1}^{*}H\; \omega_{1}} & \cdots & {v_{1}^{*}H\; \omega_{N_{T}}} \\\vdots & \ddots & \vdots \\{v_{N_{R}}^{*}H\; \omega_{1}} & \cdots & {v_{N_{R}}^{*}H\; \omega_{N_{T}}}\end{bmatrix}_{N_{R} \times N_{T}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

In Equation (2), H represents an estimated N_(T)×N_(R) channel matrixcorresponding to N_(t) BS Tx beams and NR MS Rx beams, ν*_(i) representsa beam weight for an i^(th) MS Rx beam, ω_(j) represents a beam weightfor a j^(th) BS Tx beam, and H represents a channel value between a BSTx antenna and an MS Rx antenna.

A pair of the i^(th) MS Rx beam and the j^(th) BS Tx beam having achannel element that satisfies the condition of Equation (3) or Equation(4) is selected in step 910.

$\begin{matrix}{{CINR}_{ij} = {\frac{{{\overset{\_}{H}}_{ij}}^{*}{\overset{\_}{H}}_{ij}}{\sigma_{2}} = \left. {\frac{\omega_{j}^{*}H^{*}v_{i}v_{i}^{*}H\; \omega_{j}}{\sigma^{2}} \geq {CINR}_{Th}} \middle| {{CINR}_{ij} - {CINR}_{\max}} \middle| {\leq {\Delta \; {CINR}_{Th}}} \right.}} & {{Equation}\mspace{14mu} (3)} \\{{RSSI}_{ij} = {{{{\overset{\_}{H}}_{ij}}^{*}{\overset{\_}{H}}_{ij}} = \left. {{\omega_{j}^{*}H^{*}v_{i}v_{i}^{*}H\; \omega_{j}} \geq {RSSI}_{Th}} \middle| {{RSSI}_{ij} - {RSSI}_{\max}} \middle| {\leq {\Delta \; {RSSI}_{Th}}} \right.}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

In Equations (3) and (4), H _(ij)=ν*_(i)Hω_(j) represents a channelvalue between the i^(th) MS Rx beam and the j^(th) BS Tx beam, σ²represents an Additive White Gaussian Noise (AWGN) variance, CINR_(ij)and RSSI_(ij) represent the CINR and RSSI measurements of a referencesignal transmitted by the j^(th) BS Tx beam and received by the i^(th)MS Rx beam, CINR_(max) and RSSI_(max) represent the maximum values ofthe CINRs and RSSIs of a plurality of beam pairs,max_(i,j)(RSSI_(ij))/max_(i,j)(RSSI_(ij)), and CINR_(Th) and RSSI_(Th)represent thresholds given by the measurement/selection condition of theBS in step 904. In another exemplary embodiment, a predeterminedreference value CINR_(ref) or RSSI_(ref) may be used instead ofCINR_(max) or RSSI_(max).

In the above exemplary embodiment of the present invention, the MSselects a BS Tx beam or beam pair having a CINR or RSSI equal to orlarger than a predetermined threshold or a BS Tx beam or beam pairhaving a CINR or RSSI different from the maximum of the CINR or RSSImeasurements of a plurality of beam pairs by a predetermined thresholdor less.

In another exemplary embodiment, the MS selects a BS Tx beam or beampair having a standard CINR (or RSSI) deviation equal to or smaller thana predetermined threshold.

Std. Dev(CINR_(ij)(t))≦σ_(Th)

Std. Dev(RSSI_(ij)(t))≦σ_(RSSI)   Equation (5)

In another exemplary embodiment, the MS calculates the correlationsbetween beam pairs and selects beam pairs having a correlation equal toor smaller than a correlation threshold set by the BS.

E{h* _(ij) h _(mn)}≦ρ_(Th)   Equation (6)

According to Equation (6), the MS may select a pair of an i^(th) MS Rxbeam and a j^(th) BS Tx beam and a pair of an n^(th) MS Rx beam and ann^(th) BS Tx beam.

In another exemplary embodiment, the beam selection condition may begiven by the BS or set by the MS.

In step 912, the MS feeds back information about the selected one ormore beam pairs and the CSI (e.g., RSSIs, CINRs, etc.) of the beam pairsto the BS. The MS receives a request message requesting feedbackinformation for digital beamforming of the selected one or more beampairs from the BS in step 914. The request message may includeinformation about one or more beam pairs selected by the BS. The requestmessage may further include digital beamforming information. Herein, thedigital beamforming information includes a channel measurement andfeedback report condition for the selected beam pairs. For example, thechannel measurement and feedback report condition specifies, forexample, the maximum number of BS RF paths allocated to the MS, themaximum number of MIMO streams, and feedback information needed foranalog/digital/hybrid MIMO/BF.

In step 916, the MS determines the beam pairs selected by the BS fromthe request message and detects the channel measurement and feedbackreport condition for the selected beam pairs. The MS measures thechannels of the selected beam pairs according to the channel measurementand feedback report condition in step 918. Equation (7) below expressesan effective channel matrix measured for the selected BS Tx-MS Rx beampairs by the MS.

$\begin{matrix}{{\overset{\_}{H}}_{eff} = \begin{bmatrix}{{\overset{\_}{v}}_{1}^{*}H\; \overset{\_}{\omega_{1}}} & \cdots & {{\overset{\_}{v}}_{i}^{*}H\; \overset{\_}{\omega_{N}}} \\\vdots & \ddots & \vdots \\{{\overset{\_}{v}}_{M}^{*}H{\overset{\_}{\omega}}_{1}} & \cdots & {{\overset{\_}{v}}_{M}^{*}H\; \overset{\_}{\omega_{N}}}\end{bmatrix}_{M \times {N{({{M \leq N_{R}},{N \leq N_{T}}})}}}} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

In Equation (7), M and N represent the number of MS Rx beams and thenumber of BS Tx beams for use in communication, respectively and ν _(i)and ω _(j) represent a weight for a selected i^(th) MS Rx beam and aweight for a selected j^(th) BS Tx beam, respectively.

Subsequently, step 920 or step 922 is performed depending on whetherbeamforming is performed based on a codebook or an analog feedbackbetween the BS and the MS. The type of digital beamforming to be usedmay be determined in various manners including negotiations between theBS and the MS, an indication from the BS, a request from the MS, thecapabilities of the BS and the MS, and a system standard.

In step 920, the MS transmits feedback information representing themeasured effective channel matrix to the BS. For example, the feedbackinformation includes one of the effective channel matrix, a normalizedeffective channel matrix, a decomposed effective channel matrix, acovariance matrix H _(eff) H*_(eff) of the effective channel matrix, theelements of the effective channel matrix, and information needed toreconfigure the effective channel matrix.

In step 922, the MS estimates the hybrid beamforming MIMO/BF channelcapacities of the selected beam pairs based on the effective channelmatrix and determines a necessary digital beamforming configuration.More specifically, the MS selects a MIMO mode, the number of MIMOstreams, and a preferred PMI, and estimates a CQI. The MIMO mode may beone of MIMO Tx diversity, MIMO-BF, MIMO-SM, and MIMO-STC. The estimatedCQI may be at least one of a CINR, an RSSI, an effective CINR, and anMCS level. In step 924, the MS transmits to the BS feedback informationincluding the determined digital beamforming information, for example,at least one of an optimum MIMO mode, a preferred PMI representing anoptimum digital precoder, an optimum codebook, the number of MIMOstreams, an estimated CQI, and an estimated MCS level.

The MS estimates the channel capacities of different MIMO modes havingdifferent digital MIMO precoders (codebooks) over the selected beampairs, selects at least one MIMO mode having the largest channelcapacity (or satisfying a given condition), selects a preferred PMI(precoder or codebook) for digital beamforming of the selected beampairs, and estimates CSI (a CQI, CINR, RSSI or MCS level), therebygenerating digital beamforming information in step 922.

In step 926, the MS receives a data burst from the BS using aconfiguration allocated by scheduling of the BS based on the feedbackinformation reported in step 920 or 924.

In an alternative exemplary embodiment, after selecting the beam pairsin step 910, the MS may proceed directly to step 918 and thus maymeasure the channels of the beam pairs selected by the MS in step 918.In this case, the MS completes beam selection for analog beamforming.

In another exemplary embodiment, upon request of the BS or autonomously,the MS may select BS RF chains or antenna arrays and BS Tx analog beamsor Tx-Rx analog beam pairs to be transmitted through the BS RF chains orantenna arrays and at the same time, the MS may select a MIMO rank and aMIMO precoder/codebook for use in digital MIMO precoding/beamformingthrough multiple beams.

In the above-described hybrid Tx/Rx beamforming scheme, the number ofTx/Rx beams or Tx-Rx beam pairs to be considered during channelmeasurement and selection of analog beams is changed depending on thehybrid beamforming structures of the BS and the MS and the maximumnumber of multiple analog beams that can be used simultaneously duringdigital precoding is limited by the hybrid beamforming structures of theBS and the MS. Accordingly, the MS and the BS share information abouttheir hybrid beamforming structures or their hybrid beamformingcapabilities including the capabilities of analog beamforming anddigital beamforming.

Table 1 lists exemplary hybrid beamforming capability fields accordingto hybrid beamforming structures. The capability fields include at leastone of a field indicating support or non-support of analog beamformingfor transmission and reception, a field indicating support ornon-support of digital MIMO/BF, a field indicating the number of analogTx/Rx beams in different directions, and a field indicating the numberof analog Tx/Rx beams (or the number of Tx/Rx RF paths) supported at thesame time.

TABLE 1 Category Description Value Tx Hybrid BF Capability to support Txanalog 0b0: non-support capability (in case of beamforming 0b1: supportdownlink for BS, in Number of separate Tx 0b000: 1~0b111: 8 case ofuplink for MS) arrays/subarrays Number of Tx analog beamforming- 0b0000:1~0b11111: 32 patterns (or beams) per array/subarray analog beam(s)Capability to support Tx digital 0b0: non-support MIMO/BF 0b1: supportMax number of Tx digital MIMO 0b000: 1~0b111: 8 streams supportedstream(s) Capability of transmitting concurrently 0b0: non-support withmultiple different Tx analog 0b1: support beams Max number of Tx analogbeams 0b000: 1~0b111: 8 supported concurrently (Number of Tx RF chains)Rx Hybrid BF Capability to support Rx analog 0b0: non-support capability(in case of beamforming 0b1: support uplink for BS, in Number ofseparate Rx 0b000: 1~0b111: 8 case of downlink for MS) arrays/subarraysNumber of Rx analog beamforming- 0b0000: 1~0b11111: 32 patterns (orbeams) per array/subarray analog beam(s) Capability to support Rxdigital 0b0: non-support MIMO/BF 0b1: support Max number of Rx digitalMIMO 0b000: 1~0b111: 8 streams supported stream(s) Capability ofreceiving concurrently 0b0: non-support with multiple different Rxanalog 0b1: support beams Max number of Rx analog beams 0b000: 1~0b111:8 supported concurrently (Number of Rx RF chains)

The hybrid beamforming capability fields may be shared between the BSand the MS by exchanging messages used for capability negotiationsduring network entry, handover, wake-up from idle state, or networkreentry. In another exemplary embodiment, the BS may transmit the hybridbeamforming capability fields on a broadcast message carrying commonsystem information to MSs within the cell or on a unicast messagedirected to an individual UE. In another exemplary embodiment, the BSmay include the hybrid beamforming capability fields in a requestmessage requesting a CSI, CQI or MIMO feedback, transmitted to anindividual MS.

As is apparent from the above description, exemplary embodiments of thepresent invention can mitigate a large propagation loss in amillimeter-wave band and maximize efficiency and diversity by additionaluse of MIMO/BF by performing an efficiency hybrid beamforming scheme inwhich one or more best beams are selected from a set of one or moreanalog beams having directivity on uplink/downlink and digital MIMO/BFis performed in the selected beams during transmission and receptionbetween an MS and a BS in an analog and digital hybrid beamformingstructure.

At this point it should be noted that the exemplary embodiments of thepresent disclosure as described above typically involve the processingof input data and the generation of output data to some extent. Thisinput data processing and output data generation may be implemented inhardware or software in combination with hardware. For example, specificelectronic components may be employed in a mobile device or similar orrelated circuitry for implementing the functions associated with theexemplary embodiments of the present invention as described above.Alternatively, one or more processors operating in accordance withstored instructions may implement the functions associated with theexemplary embodiments of the present invention as described above. Ifsuch is the case, it is within the scope of the present disclosure thatsuch instructions may be stored on one or more processor readablemediums. Examples of the processor readable mediums include Read-OnlyMemory (ROM), Random-Access Memory (RAM), CD-ROMs, magnetic tapes,floppy disks, and optical data storage devices. The processor readablemediums can also be distributed over network coupled computer systems sothat the instructions are stored and executed in a distributed fashion.Also, functional computer programs, instructions, and instructionsegments for accomplishing the present invention can be easily construedby programmers skilled in the art to which the present inventionpertains

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A communication method of a receiver using analog and digital hybrid beamforming, the method comprising: receiving, from a transmitter, information about at least one transmission beam selected by the transmitter; estimating a channel for the at least one transmission beam selected by the transmitter; determining feedback information comprising information on a rank and a precoding for digital beamforming based on the channel estimation result; transmitting the feedback information to the transmitter; and receiving, from the transmitter, data being scheduled based on the feedback information.
 2. The method of claim 1, wherein each of the at least one transmission beam is formed by at least one radio frequency (RF) chain or antenna array selected from among a plurality of RF chains or antenna arrays of the transmitter.
 3. The method of claim 1, further comprising transmitting, to the transmitter, report information including first information indicating at last one selected transmission RF chain or antenna array, and second information indicating at least one transmission beam selected for each of the at least one selected transmission RF chain or antenna array.
 4. The method of claim 1, wherein the feedback information for digital beamforming comprises information about at least one of one or more transmission RF chains, one or more transmission antenna arrays, a MIMO mode, a MIMO rank, the number of MIMO streams, a MIMO precoder, a MIMO codebook, channel quality information, and a modulation and coding scheme (MCS) level.
 5. The method of claim 4, wherein the feedback information for digital beamforming further comprises information about at least one of an effective channel matrix estimated by the channel estimation result, a normalized effective channel matrix, a decomposed effective channel matrix, a covariance matrix of the effective channel matrix, elements of the effective channel matrix, and information required to reconfigure the effective channel matrix.
 6. The method of claim 1, further comprising receiving, from the transmitter, a message including information related to measurement for beamforming, wherein the message comprises at least one of the number of transmission RF chains to be reported, the number of transmission antenna arrays to be reported, the number of transmission beams per transmission RF chain or transmission antenna array to be reported, the number of transmission beams to be reported, the number of beam pairs to be reported, the number of transmission beams per reception beam to be reported, the number of reception beams to be reported, the number of MIMO streams, a channel quality condition used to select a transmission beam or a pair of a transmission beam and a reception beam, a MIMO mode to be fed back, a supported MIMO rank, and the number of supported MIMO streams.
 7. The method of claim 1, further comprising: selecting a transmission beam having a signal to interference and noise ratio (SINR) or signal strength equal to or larger than a threshold from among the plurality of transmission beams; selecting a transmission beam having an SINR or signal strength different from a predetermined reference value by a given relative threshold from among the plurality of transmission beams; selecting a transmission beam having an SINR standard deviation or signal strength standard deviation equal to or larger than a threshold from among the plurality of transmission beams; and selecting a pair of transmission beams having a correlation equal to or larger than a threshold from among the plurality of transmission beams.
 8. The method of claim 1, wherein the determining of the feedback information for digital beamforming comprises: determining an effective channel matrix based on a selected transmission RF chain or antenna array and a transmission beam corresponding to the selected transmission RF chain or antenna array; selecting a precoder or codebook that maximizes a signal to noise ratio (SNR) of each MIMO stream or a channel capacity, estimated with respect to a supported MIMO rank or the number of supported MIMO streams; estimating an effective carrier to interference and noise ratio (CINR) according to the selected precoder or codebook; and generating the feedback information for digital beamforming with information about at least one of the selected precoder or codebook and the effective CINR.
 9. A communication method of a transmitter using analog and digital hybrid beamforming, the method comprising: determining at least one transmission beam for a receiver; transmitting, to the receiver, information about the at least one transmission beam for the receiver; receiving, from the receiver, feedback information comprising information on a rank and a precoding for digital beamforming, the feedback information being determined based on a channel estimation result on the at least one selected transmission beam for the receiver; and transmitting, to the receiver, data being scheduled based on the feedback information.
 10. The method of claim 9, wherein each of the at least one transmission beam is formed by at least one radio frequency (RF) chain or antenna array selected from among a plurality of RF chains or antenna arrays of the transmitter.
 11. The method of claim 9, further comprising receiving, from the receiver, report information including first information indicating at least one selected transmission RF chain or antenna array, and second information indicating at least one transmission beam selected for each of the at least one selected transmission RF chain or antenna array.
 12. The method of claim 9, wherein the feedback information for digital beamforming comprises information about at least one of one or more transmission RF chains, one or more transmission antenna arrays, a MIMO mode, a MIMO rank, the number of MIMO streams, a MIMO precoder, a MIMO codebook, channel quality information, and a modulation and coding scheme (MCS) level.
 13. The method of claim 12, wherein the feedback information for digital beamforming further comprises information about at least one of an effective channel matrix estimated by the channel estimation result, a normalized effective channel matrix, a decomposed effective channel matrix, a covariance matrix of the effective channel matrix, elements of the effective channel matrix, and information required to reconfigure the effective channel matrix.
 14. The method of claim 9, further comprising transmitting, to the receiver, a message including information related to measurement for beamforming, wherein the message includes at least one of the number of transmission RF chains to be reported, the number of transmission antenna arrays to be reported, the number of transmission beams per transmission RF chain or transmission antenna array to be reported, the number of transmission beams to be reported, the number of beam pairs to be reported, the number of transmission beams per reception beam to be reported, the number of reception beams to be reported, the number of MIMO streams, a channel quality condition used to select a transmission beam or a pair of a transmission beam and a reception beam, a MIMO mode to be fed back, a supported MIMO rank, and the number of supported MIMO streams.
 15. A mobile station (MS) for performing communication using analog and digital hybrid beamforming, the MS comprising: a transceiver; and at least one processor configured to control digital beamforming and analog beamforming of the transceiver, wherein the at least one processor is configured to: receive, from a transmitter, information about at least one transmission beam selected by the transmitter, estimate a channel for the at least one transmission beam selected by the transmitter, determine feedback information comprising information on a rank and a precoding for digital beamforming based on the channel estimation result, transmit, to the transmitter, the feedback information, and receive, from the transmitter, data being scheduled based on the feedback information.
 16. The mobile station of claim 15, wherein each of the at least one transmission beam is formed by at least one radio frequency (RF) chain or antenna array selected from among a plurality of RF chains or antenna arrays of the transmitter.
 17. The mobile station of claim 15, wherein the at least one processor is further configured to transmit, to the transmitter, report information including first information indicating at last one selected transmission RF chain or antenna array, and second information indicating at least one transmission beam selected for each of the at least one selected transmission RF chain or antenna array.
 18. The mobile station of claim 15, wherein the feedback information for digital beamforming comprises information about at least one of one or more transmission RF chains, one or more transmission antenna arrays, a MIMO mode, a MIMO rank, the number of MIMO streams, a MIMO precoder, a MIMO codebook, channel quality information, and a modulation and coding scheme (MCS) level.
 19. A base station (BS) for performing communication using analog and digital hybrid beamforming, the BS comprising: a transceiver; and at least one processor configured to control digital beamforming and analog beamforming of the transceiver, wherein the at least one processor is configured to: determine at least one transmission beam for a receiver, transmit, to the receiver, information about the at least one transmission beam, receive, from the receiver, feedback information comprising information on a rank and a precoding for digital beamforming, the feedback information being determined based on a channel estimation result on the at least one selected transmission beam, and transmit data being scheduled based on the feedback information.
 20. The base station of claim 19, wherein each of the at least one transmission beam is formed by at least one radio frequency (RF) chain or antenna array selected from among a plurality of RF chains or antenna arrays of the transmitter.
 21. The base station of claim 19, wherein the at least one processor is further configured to receive, from the receiver, report information including first information indicating at least one selected transmission RF chain or antenna array, and second information indicating at least one transmission beam selected for each of the at least one selected transmission RF chain or antenna array.
 22. The method of claim 19, wherein the feedback information for digital beamforming comprises information about at least one of one or more transmission RF chains, one or more transmission antenna arrays, a MIMO mode, a MIMO rank, the number of MIMO streams, a MIMO precoder, a MIMO codebook, channel quality information, and a modulation and coding scheme (MCS) level. 